Initially I started with the full hard roof and two piece windshield on my P4. I noticed right away that there was quite a bit of back draft air coming in the cab from behind the front seats bringing in lots of dust and snow into the cab. One thing nice I noticed about the two piece windshield is that if you adjust it open to allow a little air in up front, you can off set this back draft quite a bit. Problem with doing that in cold weather conditions however are obvious. If you are using your machine in a warm weather climate, you might be fine.
Because I live in Minnesota, and do quite a bit of plowing with my machine, I decided to add the hard mid panel around December and was extremly supprised what a differance it made. With the windshield now closed, even at moderate to high speeds, the air inside the cab became very stable and comfortable even with a modest cross wind. I found that there was little to no air movement inside which actually keeps the dust and snow out.
I primarily use my pioneer for utility and don't do as much trail riding as some others here from the site. I would think the results would be similar however.
Additionally, like allot of us here who are building our machines as we go, I decided to wait on doors and may or may not even need them down the road.